OriginPeriod: Organizes time periods

#' @include Validation.R
#' 
#' @title OriginPeriod-class
#' 
#' @description
#' 
#' OriginPeriod is an S4 class used to store information about the time 
#' intervals which produces claims, exposure or some other measure. This is 
#' a utility class, primarily of interest to actuaries.
#' 
#' @details
#' Several things distinguish an OriginPeriod from other date/time vectors.
#' 
#' 1. A starting and ending date are both required.
#' 2. The time period between the starting and ending date must be constant.
#' 3. The start and end dates must be monotonically increasing
#' 4. The user may apply a label to each period
#' 4. The time periods may not overlap (?)
#' 
#' An OriginPeriod may be of any arbitrary length, though years are most 
#' common. The "type" slot is a character string which indicates the type 
#' of origin period, such as "Accident", "Occurrence", "Report", "Policy", 
#' "Lloyds" and so forth. "Accident" and "Occurrence" are synonymous and 
#' indicate the a claim occurred in a particular period of time. "Report" 
#' is the interval during which a claim is reported, such as exists under 
#' a claims-made policy. "Policy" or "Underwriting" periods are ones wherein 
#' the risk attaches. "Lloyds" refers to a Lloyds year of account. There is 
#' no restriction on what type may be used for an OriginPeriod, meaning that 
#' there is no language restriction either. So, "Zeichnungsjahr", 
#' "Ereignisjahr" and so forth are permitted.
#' 
#' 
#' @name OriginPeriod-class
#' @rdname OriginPeriod-class
#' @exportClass OriginPeriod
#' 
#' @importClassesFrom lubridate Period
setClass("OriginPeriod"
         , representation(StartDate = "Date"
                          , EndDate = "Date"
                          , Moniker = "character"
                          , Period = "Period"
                          , Type = "character")
         , validity = checkOriginPeriod
)

#' is.OriginPeriod
#' 
#' Checks whether the object is an object of the OriginPeriod class.
#' 
#' @param object Object to be tested
#' 
#' @return TRUE/FALSE
#' 
#' @usage is.OriginPeriod(object)
#' @export
#' 
is.OriginPeriod = function(object)
{
  is(object, "OriginPeriod")
}